Just had my truck at the dealer today for same issue. They can't figure it out. Only thing that I had done 2 months ago was replace water pump and timing belt which they checked today and everything was fine. Squealing starts when letting up off the accelerator but is not consistent.

Try checking your exhaust brackets, sometimes if they're worn/bent, or not in the exact same position, you'll get a metallic squealing noise from your undercarriage, especially if you've had the exhaust apart, it happened to my '01 and '04's prior to replacing the whole system.